Method for cooling a moving metal belt

ABSTRACT

The present invention relates to a method for cooling a moving metal belt ( 2 ) in a continuous processing line by spraying a gas, a liquid, or a mixture consisting of gas and liquid onto the belt, the processing line including a cooling section ( 1 ) followed by a downstream section ( 4 ), the inlet ( 4   a ) of the downstream section corresponding to the outlet ( 1   b ) of the cooling section, wherein according to said method: the change in the temperature cross-section of the belt between the inlet ( 4   a ) and the outlet ( 4   b ) of the downstream section ( 4 ) is evaluated; the temperature cross-section suitable for the inlet of the downstream section is deduced, on the basis of a desired temperature cross-section at the outlet of the downstream section ( 4 ), in order to obtain the desired cross-section at the outlet; and the cooling capacity of the cooling section ( 1 ) is adjusted according to the width of the belt and over the length of the cooling section, while taking into the account the temperature cross-section of the belt at the inlet of the cooling section, so that the cooling makes it possible to obtain the aforementioned temperature cross-section at the outlet of the cooling section.

The present invention relates to improvements made to the cooling sections of continuous processing lines for metal strips, in particular annealing, galvanization or tinplate lines.

A continuous processing line for metal strips is made up of a succession of heat treatment sections, in particular heating, maintenance, cooling, ageing sections, etc.

The present invention relates to cooling sections of continuous processing lines and more particularly to the rapid cooling sections, whatever the cooling method used, for example radiation, convection or any other cooling method.

The cooling of the metal strip can be obtained by blowing a gas, for example air, but more generally a mixture of nitrogen and hydrogen, onto the strip. The hydrogen content of the mixture is generally at least equal to 5%, so as to limit the oxidation of the strip. Higher hydrogen contents are frequently used to improve cooling performances, by virtue of the gain in exchange coefficient resulting from the physical properties of the hydrogen.

In order to obtain even greater cooling gradients for the strip, the cooling can equally be obtained by spraying a liquid onto the strip. This liquid is frequently water, which can be pretreated, for example in order to extract dissolved oxygen or mineral salts therefrom, and which can contain additives in order to improve the heat exchange or limit the oxidation of the strip.

The cooling of the strip can equally be obtained by spraying a mixture consisting of gas and liquid onto the strip. The gas which is used is generally nitrogen, but can equally be composed of a mixture of nitrogen and hydrogen, or any other gas. The cooling liquid is frequently water, possibly treated as previously described.

The quality of the cooling has a substantial impact on the mechanical properties of the strip and on its surface finish. According to the starting and end cooling temperatures and the cooling gradients, the cooling of a metal strip is generally accompanied by metallurgical transformations with phase changes so as to obtain the sought mechanical properties, for example in terms mechanical resistance or drawability. The nature of the formed phases, their proportion and their morphology depend on the temperatures and the cooling gradients. Good temperature homogeneity over the length of the strip along the cooling section is thus crucial to ensuring that the obtained metallurgical transformations are those which are sought.

The continuous processing lines have high strip running speeds for the strip, for example from 100 to 800 m/min, the strip circulating on transport rollers. The guidance of the moving strip in the different sections of the line is crucial to avoiding contact between the strip and the walls. Differences in length over the width of the strip, with, for example, long or short edges relative to the center of the strip, influence the quality of guidance of the strip. It will be appreciated that a difference in cooling intensity over the width of the strip leads to a difference in temperature and hence a difference in contraction of the strip over its width, which impacts on the guidance of the strip.

Following its exit from the cooling section, the strip circulates in the downstream-situated sections, where the thermal path of the strip continues. It can, for example, cross an ageing section, with the latter being maintained at an appropriate temperature for a period of several seconds to several minutes.

In the course of its passage into the section of the furnace situated downstream of the cooling section, the strip will see its average temperature develop with a temperature rise in the case of a warming section, or a temperature drop in the case of a cooling section. The average temperature of the strip will likewise be able to be kept constant in respect of a maintenance section. According to the nature, the geometry, the means used for the warming or cooling and the method of control of the section situated downstream of the cooling section, the transverse temperature profile of the strip will be able to develop between the inlet and outlet of this section by virtue of a different heat exchange over the strip width. Thus, a strip which is perfectly homogeneous in temperature at exit from the cooling section will be able to have warmer or cooler edges than the center at the outlet of the downstream chamber. It will be readily appreciated that the temperature profile of the strip at the outlet of the section situated downstream of the cooling section will equally be linked to the temperature profile of the strip at exit from the cooling section. It is hence possible to influence the temperature profile of the strip at exit from the downstream section according to the temperature profile of the strip at exit from the cooling section.

It is evident that the control of the cooling over the strip width, over the entire length of the cooling section, is critical to obtaining homogeneous mechanical properties over the strip width, to avoiding strip guidance defects and to anticipating the development of the temperature profile of the strip in the downstream-situated section. This control is particularly crucial for wide and relatively thin strips.

The object of the invention is, above all, to improve the control of the cooling over the strip width in order to meet these requirements, so that the cooling curve at any point on the width of the strip along the cooling section is that which is sought.

The invention thus relates to a method for cooling a moving metal strip in a continuous processing line by spraying a gas, a liquid, or a mixture consisting of gas and liquid onto the strip, the processing line comprising a cooling section followed by a downstream section having a thermal effect upon the strip, the inlet of the downstream section corresponding to the outlet of the cooling section.

The method of the invention is characterized in that:

-   -   the change in the transverse temperature profile of the strip         between the inlet and the outlet of the downstream section is         evaluated in real time by means of a computer on the basis of         mathematical models as a function of the format of the strip,         the running speed and the transverse temperature profile of the         strip at the inlet of the downstream section,     -   from a desired transverse temperature profile at exit from the         downstream section is deduced that transverse temperature         profile at the inlet of the downstream section which is suitable         for obtaining the desired exit profile,     -   and the cooling capacity of the cooling section is adjusted         according to the width of the strip and over the length of the         cooling section in real time by a control and operating system         of the line, by means of the computer, on the basis of         mathematical models, while taking into account the transverse         temperature profile of the strip at the inlet of the cooling         section and the development of the heat exchanges between the         strip and its environment in the cooling section, so that the         cooling makes it possible to obtain the aforementioned suitable         transverse temperature profile at exit from the cooling section.

According to the invention, the cooling capacity is thus adjusted, while taking into account, in advance, the future development of the transverse temperature profile of the strip during its stay in that section of the line situated downstream of the cooling section.

The control of the temperature profile over the width of the strip resulting from the adjustment of the cooling capacity over the strip width is designed to allow the guidance of the strip on the transport rollers to be improved by the obtention of long or short edges relative to the center of the strip.

The adjustment of the cooling capacity can be obtained by splitting the cooling device into a plurality of units along the width and along the length of the cooling section. Each unit can be provided with regulating members in order to vary its cooling capacity independently from the other units.

The operation of the regulating members can be carried out from a computer, in which is installed an appropriate control program for the cooling units. Advantageously, the computer receives data supplied by temperature sensors distributed in the cooling section and by temperature sensors distributed in the downstream section, and the computer, on the basis of these data, checks whether the cooling is effected in the desired manner, and possibly corrects the execution of the cooling according to the width of the strip and according to its length in order to obtain the desired profile.

Aside from the arrangements set out above, the invention consists in a certain number of other arrangements which will be discussed more explicitly below in connection with an illustrative embodiment described with reference to the appended drawing, but which is by no means restrictive. In this drawing:

FIG. 1 is a schematic vertical section of a cooling section and of a downstream section of a continuous processing line for a metal strip.

FIG. 2 is a horizontal section of the cooling section along the line II-II of FIG. 1, and

FIG. 3 is a graph illustrating the variations in the transverse temperature profile of the strip (y-axis) plotted against the width of the strip (y-axis).

Referring to FIGS. 1 and 2 of the drawing, a cooling section 1 of a continuous processing line for a moving metal strip 2 can be seen. According to the represented example, the cooling section 1 is vertical, but it could also be horizontal, or inclined relative to the vertical. In the top portion of the section 1, the strip 2 passes over return rollers 3 so as to engage in a downstream section 4 likewise shown vertical in the example, but which can be arranged otherwise, especially horizontally. The width (FIG. 2) of the strip 2 is perpendicular to the plane of FIG. 1.

The cooling of the strip 2 is carried out by spraying a gas, a liquid, or a mixture consisting of gas and liquid, onto each of the faces of the strip, with the aid of nozzles 5 distributed in the walls of the section 1 which stand parallel to the strip 2 on each side of this strip. The nozzles 5 are oriented in such a way as to direct at least one jet of cooling fluid against the strip 2, especially in a direction substantially at right angles to this strip. The nozzles 5 are fed with cooling fluid through the pipes 6.

The inlet 4 a of the downstream section corresponds to the outlet 1 b of the cooling section. The strip 2 has a transverse temperature profile P (FIG. 3) which is dependent on the considered zone of the strip 2. The profile P represents the variation in temperature of a point of the strip according to its width, which corresponds to a direction at right angles to the direction of displacement of the strip.

According to the invention, the change in transverse temperature profile between the inlet 4 a and the outlet 4 b of the downstream section is evaluated as a function of the format of the strip 2, in particular according to its width, its thickness and its nature, and as a function of the transverse temperature profile of the strip at the inlet 4 a of the downstream section.

This change in transverse profile can be evaluated on the basis of mathematical models by which the heat exchanges between the strip 2 and the section 4 can be calculated, possibly supplemented by prior measures.

Next, by a reverse step, on the basis of a desired transverse temperature profile P4 b (FIG. 3) at exit 4 b from the downstream section, the transverse temperature profile P4 a, at the inlet 4 a of the downstream section, which is suitable for obtaining the desired exit profile P4 b, is deduced.

In the schematic example of FIG. 3, the desired exit profile P4 b is a homogeneous profile according to width, that is to say that the temperature of the strip is constant from one edge to the other. In this example, the downstream section 4 subjects the strip to a warming which is more pronounced on the left-hand edge than at the center and on the other edge. The profile P4 a at the inlet of the section, suitable for giving the profile P4 b, will have an upwardly convex shape on the left edge, corresponding to a lesser strip temperature on the left edge.

On the one hand, the transverse profile P4 a at the inlet of the downstream section 4 a, which is also the profile P1 b at the outlet 1 b of the cooling section, is at hand, and, on the other hand, the transverse temperature profile P1 a at the inlet of the cooling section is known. According to the example of FIG. 3, the profile P1 a is upwardly concave, corresponding to strip edges warmer than the center. The cooling capacity of the cooling section 1 is then adjusted according to the width of the strip 2 and according to the length of the cooling section, in order to obtain, from the entry profile P1 a, the exit profile P1 b=P4 a. The transverse temperature profile P1 a at the inlet is known with the aid of temperature sensors distributed over the width of the strip, at the inlet 1 a.

The adjustment of the cooling capacity over the width of the strip can be obtained by numerous known means.

Advantageously, this adjustment of the cooling capacity is obtained by splitting a cooling device R into a plurality of units Ryz along the width and along the length of the cooling section 1, that is to say in the vertical direction according to the considered example. The value y of Ryz can vary from 1 to m, m being the number of units according to the width of the strip, while the value z can vary from 1 to n, n being the number of units according to the length of the cooling section 1.

Generally speaking, each unit Ryz is equipped with a member, for example a regulator valve 7, by which the flow of the cooling means, gas, liquid, or gas/liquid mixture can be varied. In the case of a gas/liquid mixture, a regulator valve can be necessary in respect of each fluid. Each unit is thus provided with the equipment necessary to vary its cooling capacity independently from the other units.

In the example illustrated in FIG. 1, each cooling unit Ryz comprises two nozzles 5, having the same position according to width, but vertically staggered according to length. The nozzles 5 of a same unit are fed in parallel from a same pipe 6, on which is disposed a regulator valve 7 controlling the flow of gas or the flow of the cooling liquid.

The operation of the regulating members, such as the valves 7, is carried out from a computer A, in which is installed an appropriate operating program for the cooling units. The computer A additionally receives data supplied by temperature sensors 8 distributed in the cooling section and by temperature sensors 9 distributed in the downstream section. On the basis of these data, the computer A checks whether the cooling is effected in the desired manner, and possibly corrects the execution of the cooling, according to the width of the strip and according to its length, in order to obtain the desired profile.

Where cooling is effected by a mixture consisting of a gas and a liquid, each unit will be able to be equipped, for example, with a flow control member solely in relation to the gas, the flow of the liquid being constant, or with a flow control member solely in relation to the liquid, the flow of the gas being constant, or with two control members, by which the flow of gas and the flow of the liquid can be varied. Each unit can equally be equipped with a device G, by which the temperature of the gas, of the liquid, or of the mixture consisting of gas and liquid can be varied, so as to vary its cooling capacity. This variation of the temperature of the cooling means will be able to be realized for a constant flow of the cooling means, or combined with a variation of the flow of the cooling means so as to increase the regulating flexibility of the installation.

By way of example:

-   -   if the section 4 situated downstream of the cooling section 1 is         a warming section which leads to a higher temperature of one of         the edges of the strip of 5° C., for example the left edge,         whereas a homogeneous temperature at exit 4 b from said section         is sought,     -   and if the strip 2 enters into the cooling section 1 with         perfect homogeneity of temperature over its width,

then, according to the invention, the cooling parameters are adjusted such that a greater cooling capacity on the considered edge, the left edge in the example, leads to a supplementary cooling of this latter of 5° C. relative to the rest of the width of the strip.

According to a variant of this example:

-   -   if the strip 2 enters at present into the cooling section 1 with         the considered edge 10° C. cooler than the rest of the strip,

then, according to the invention, the cooling parameters are adjusted so that a weaker cooling capacity on the considered edge leads to a lesser cooling of this latter of 10° C. relative to the rest of the strip width.

The program or programs installed in the computer A are established with mathematical means, utilizing models based on the physical laws of heat exchanges, and allow good simulation of the temperature variations of a strip 2, when passing into a section of a continuous line, according to the nature of said strip and its thermal state. It is hence possible to predict the development of the temperature profile of the strip along this section and consequently to adjust the working parameters of each unit of the cooling section.

Tests conducted at the time of the start-up of the continuous line are likewise put to use to refine the thermal model and increase the precision of the device, by improving the program installed in the computer. 

1. A method for cooling a moving metal strip in a continuous processing line by spraying a gas, a liquid, or a mixture consisting of gas and liquid onto the strip, the processing line comprising a cooling section followed by a downstream section having a thermal effect upon the strip, the inlet of the downstream section corresponding to the outlet of the cooling section, wherein: the change in the transverse temperature profile of the strip between the inlet and the outlet of the downstream section is determined in real time by means of a computer on the basis of mathematical models, while taking into account the format of the strip, the running speed of the strip, the transverse temperature profile of the strip at the inlet of the downstream section, and the development of the heat exchanges between the strip and its environment in the downstream section, on the basis of a desired transverse temperature profile at exit from the downstream section, the transverse temperature profile at the inlet of the downstream section is suitable for obtaining the desired exit profile, the cooling capacity of the cooling section is regulated according to the width of the strip and over the length of the cooling section in real time by a control and operating system of the line, by means of the computer, on the basis of mathematical models, while taking into account the transverse temperature profile of the strip at the inlet of the cooling section and the development of the heat exchanges between the strip and its environment in the cooling section, so that the cooling makes it possible to obtain the aforementioned suitable transverse temperature profile at exit from the cooling section.
 2. The method as claimed in claim 1, wherein the adjustment of the cooling capacity is obtained by splitting a cooling device into a plurality of units along the width and along the length of the cooling section.
 3. The method as claimed in claim 2, wherein each unit is provided with regulating members in order to vary its cooling capacity independently from the other units.
 4. The method as claimed in claim 3, wherein the operation of the regulating members is carried out from the computer, in which is installed an appropriate operating program for the cooling units.
 5. The method as claimed in claim 4, wherein the computer receives data supplied by temperature sensors distributed in the cooling section and by temperature sensors distributed in the downstream section, and the computer, on the basis of these data, checks whether the cooling is effected in the desired manner, and possibly corrects the execution of the cooling according to the width of the strip and according to its length in order to obtain the desired profile. 